Bipolar Transistor Trench Isolation with Inclined Sidewalls

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Solution Overview

Problem

Conventional bipolar junction transistors face challenges in enhancing device performance for high-frequency applications, particularly in radiofrequency integrated circuits and BiCMOS integrated circuits, due to limitations in device structure and fabrication methods.

Innovation Solution

A method for fabricating bipolar junction transistors involving the formation of a collector region and an intrinsic base layer on a semiconductor substrate, with trench isolation regions and anisotropic etching processes to create inclined sidewalls, enhancing the device structure and reducing parasitic capacitance.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If conventional bipolar junction transistor structures are used, then device fabrication is simpler, but collector-base capacitance is higher and cut-off frequency is lower

Engineering Contradiction:

Improvedevice performanceVSAvoiddevice structure

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The device structure is segmented into distinct regions with different sidewall configurations. The isolation region is divided into a first portion (in the base region) and a second portion (in the collector region), each with independently configured sidewalls. This segmentation allows optimized electrical characteristics in different zones, reducing collector-base capacitance while maintaining manufacturability through modular structure design.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

Different regions of the device are given different local properties. The first sidewalls in the base region have one configuration (e.g., vertical or angled at a first angle), while the second sidewalls in the collector region have a different configuration (e.g., angled at a second angle). This local differentiation optimizes charge carrier control and reduces parasitic capacitance in the critical collector-base junction area without compromising overall device performance.

Inventive Principle:

Principle #3Local quality

2Reliability

If standard isotropic etching is used, then manufacturing process is simpler, but parasitic capacitance cannot be reduced effectively

Engineering Contradiction:

Improvecut-off frequencyVSAvoidfabrication process

Core Design Contradiction:

ReliabilityVSEase of manufacture

Solution Approach 1:

The etching process parameters are changed from isotropic to anisotropic etching. This parameter change enables the formation of inclined sidewalls with specific angles in the collector region, which reduces the overlap area between the base and collector regions, thereby reducing parasitic capacitance and increasing cut-off frequency. The anisotropic etching provides directional material removal that is critical for achieving the desired sidewall geometry.

Inventive Principle:

Principle #35Parameter changes

3Reliability

If vertical sidewalls are used throughout, then fabrication is easier, but heat transfer optimization is limited

Engineering Contradiction:

Improveheat transferVSAvoidsidewall configuration

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The device structure employs asymmetric sidewall configurations. The first sidewalls in the base region may be vertical or have a first angle, while the second sidewalls in the collector region have a different angle. This asymmetry optimizes heat transfer pathways by creating favorable thermal conduction paths from the collector region to the substrate, while also optimizing electrical characteristics. The different sidewall angles in different regions provide tailored thermal management without requiring complete structural redesign.

Inventive Principle:

Principle #4Asymmetry

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

The approach improves the performance of bipolar junction transistors by reducing collector-base capacitance, increasing cut-off frequency, and optimizing heat transfer, thereby enhancing the overall performance for high-frequency applications.

Implementation Method 1

etching the collector region adjacent to the second section of the trench with a second etching process that etches different crystallographic directions of the single-crystal semiconductor material of the substrate at different etch rates

Methodology Applied

Scientific EffectAnisotropic etching:

Data Source

PatentUS9093491B2Bipolar junction transistors with reduced base-collector junction capacitance

Publication Date: 2015.07.28 GLOBALFOUNDRIES US INC

AI summary

Fabrication methods, device structures, and design structures for a bipolar junction transistor. The device structure includes a collector region, an intrinsic base formed on the collector region, an emitter coupled with the intrinsic base and separated from the collector by the intrinsic base, and an isolation region extending through the intrinsic base to the collector region. The isolation region is formed with a first section having first sidewalls that extend through the intrinsic base and a second section with second sidewalls that extend into the collector region. The second sidewalls are inclined relative to the first sidewalls. The isolation region is positioned in a trench that is formed with first and second etching process in which the latter etches different crystallographic directions of a single-crystal semiconductor material at different etch rates.